Why do I keep seeing references to a fuel tank capacity limit? There is none!
Read through the regs to see for yourself: Section 5 details the fuel flow rate at 100kg/h (a mass flow rate, as it should be, not the quoted volumetric rate of 140L/h), and section 6 makes no restrictions on the size of the fuel tank.
http://www.fia.com/sites/default/files/ ... 111212.pdf
As for your last question, the regs allow for a clutch and do not disallow gearing of the MGHU, but do specify a maximum RPM limit on the motor, but not the turbine shaft speed itself:
5.2.4 The MGU-H must be solely mechanically linked to the exhaust turbine of a pressure charging system. This mechanical link must be of fixed speed ratio to the exhaust turbine and may be clutched.
The rotational speed of the MGU-H may not exceed 125,000rpm.